From steinmetz!uunet!seismo!sundc!pitstop!sun!decwrl!hplabs!hpda!hpsal2!hpcupt1!hpirs!hpisoa2!jonathan Tue Feb  2 21:36:40 1988
Path: beowulf!steinmetz!uunet!seismo!sundc!pitstop!sun!decwrl!hplabs!hpda!hpsal2!hpcupt1!hpirs!hpisoa2!jonathan
From: jonathan@hpisoa2.HP.COM (Jonathan Hue)
Newsgroups: comp.graphics
Subject: Re: RGB to printer CMYK conversion
Message-ID: <1090001@hpisoa2.HP.COM>
Date: 3 Feb 88 02:36:40 GMT
References: <10258@sgi.SGI.COM>
Organization: Hewlett Packard, Cupertino
Lines: 44

>Someone asked about converting from RGB to prinyer CMYK.  Here is a
>simple conversion technique.

...which doesn't really produce anything useful.  If I had to do this,
(which I don't) here is what I would do:

Let's assume you have your system calibrated so that you can output
to a good film recorder (Dunn, Celco, Genigraphics, etc, but not a QCR)
and your transparency pretty much looks like your monitor.  Decide
what kind of paper and ink you are going to use.  Then, make a
Scitex CT2T magnetic tape of a bunch of color squares.  What would
probably be useful are the CMYK quads from (0,0,0,0) to (255,255,255,255)
in increments of 16.  Have a pre-press shop make a Cromalin from the
tape.  Next, make a transparency consisting of the RGB triplets from
(0,0,0) to (255,255,255) in little squares on your film recorder.  Then use
a transmission-reflection densitometer and measure the densities of your
17^3 RGB triplets and 17^4 CMYK quads.  Use a densitometer with an RS-232
on it so you don't have to write down all those numbers.  Interpolate the
17 levels of each color to some reasonable number, perhaps a number near a
hundred, maybe more for the CMYK quads.  Then map the RGB numbers into
the nearest CMYK quad.  Now you have an CMYK quad (for a given type of
ink and paper which is reasonably close to the RGB value you used to make
the color spot on your transparency.

This is what I thought of doing, other people presented with the same
problem have done the same thing as it's sort of the obvious thing to
do when you don't really understand the physics and math behind the
problem too well.  Some people think the results are pretty good.

This doesn't take too much work and you get way better results than
treating the inks as a "color space" which most people seem to
want to do for some reason, rather than real pigments.  If you are
smart I'm sure you can come up with a much better way, but at least
this does something useful.  If you have a big computer you can perhaps
interpolate 256^3 RGB triplets into 256^4 CMYK quads and get some
accurate numbers, even though you can't reproduce dots on the paper that
well.

Once you get this far it should be pretty easy to calibrate your monitor
to your transparency.  A color-tv analyzer (like Minolta's) would be
useful, and if you can find it, a densitometer that reads out in XYZ
or whatever that color space is called, I forget.

Jonathan Hue		..!hpda!jonathan